Reinforced semiconductor wafer holder

ABSTRACT

An enclosed semiconductor wafer holder, and a cover providing isolation control of semiconductor wafers that is simple, lower cost, and non-obtrusive is described. The device includes a cover with a sealing perimeter that creates a hermetic seal when engaged with the semiconductor wafer holder. It also includes a purging means attached to the holder that allows an active gentle purging of inert gas into the sealed semiconductor wafer holder. The holder has contoured recess-forming pairs that do not effect semiconductor wafer transfer when the recess forming pairs are warped. The recess-forming pairs also have integrally corresponding ribs that help prevent warpage of the contoured recess-forming pairs during the plastic injection molding process.

BACKGROUND OF THE INVENTION

I. Field of the Invention

This invention relates generally to semiconductor wafer holders used inthe storage, transportation and processing of semiconductor wafers. Moreparticularly, it relates to a hermetically sealable purgable cassetteused for containing semiconductor wafers during processing,transportation and storage having recess-forming pairs for supportingsemiconductor wafers. The cassette of the present invention helps reducewarpage during the plastic injection molding and cooling process that isused in making the cassette, and during use of the cassette.

II. Discussion of the Prior Art

Production of semiconductor wafers requires an extremely cleanenvironment. The presence of any small particles, vapors or staticdischarge in the environment is damaging to the production ofsemiconductors, and the wafers themselves. In an effort to combatairborne particle problems, various techniques are in use today.

The most common technique in use today, is to provide a cassette forholding the semiconductor wafers which meet Semiconductor Equipment andMaterials International (SEMI) standards. The SEMI standards recitespecifications with the purpose of defining interchangeable,standardized containers suitable for standardized processing cassettes.A standardized mechanical interface (SMIF) system has been proposed byHewlett-Packard Company as disclosed in U.S. Pat. Nos. 4,532,970 and4,534,389 with the purpose of reducing particle fluxes on thesemiconductor wafers.

In the SMIF system, a cassette meeting SEMI standards is placed inside aclean Work In Process (WIP) box or pod. The SMIF pod or box keeps thecassette and wafers free of particles during transportation and storage.The pod or box also isolates the wafers from the operator. Thesemiconductor wafer cassette, the wafers, and the inside of the box orpod must all be free of particles damaging to the semiconductormanufacturing process.

The SMIF pods or boxes are used during processing in either a generalclean room environment or a clean mini-environment (i.e.: under acanopy). Once in a clean environment, the SMIF boxes or pods are openedfor removal of the cassette and for processing of the wafers. This canbe done in a clean environment without contaminating the cassette orwafers.

During the automated processing of the semiconductor wafers a roboticarm of the processing tool removes the wafers from an indexed cassetteand returns the processed wafers to their proper position. To preventdamage to the semiconductor wafers, when being removed and returned tothe cassette by the robotic arm, the cassette must uniformly position aplurality of wafers within the cassette according to parameterspre-programmed into the automated process tool. For example, eachwafer's horizontal alignment and the wafer's tilt is pre-programmed intothe automated processing tool. The robotic arm then advances to thisexact location to remove or return the semiconductor wafers.

When placing wafers inside the cassette, a recess-forming pair ordivider or slot-forming spacer means supports and separates the wafers apredetermined distance from each other. This distance is also known bythe robotic arm. During the molding process used to make the cassetteand during use of the cassette, these recess-forming pairs may warpthereby causing the wafers to tilt outside the parameters pre-programmedinto the automated processing tool. To remove the wafer, the robotic armaligns itself within the cassette according to the pre-programmedparameters. If the wafer is tilted or not in horizontal alignment, therobotic arm may crash into the wafer, thereby damaging or destroying thesemiconductor wafer.

Likewise, when replacing the wafer, if the divider has warped andtherefore is not in the pre-programmed position, the wafer may scrape orcrash against the warped divider. This damage or destruction to thesemiconductor wafers can prove costly. Therefore, there is a need for astandard acceptable cassette having dividers that do not effect wafertransfer when warpage occurs.

The warp and shrink of the recess-forming pairs may be caused by severalfactors including: the polymer compound used, the selected pressparameters for the plastic injection mold, part design, the total volumeof each recess-forming pair, the thickness of the cassette sides, andthe volume of the perimeter lip. Currently, with fiber filled resins,control of warping and shrinking during the plastic injection moldingprocess is very difficult, if not impossible. Even when running the moldpress within acceptable parameters, a significant percentage of moldedcassettes have warped dividers that do not fall within the tolerancesnecessary for the wafer processing. This results in an over abundance ofwarped, unacceptable cassettes in the cassette manufacturing process.The molding of unacceptable cassettes increases the total cost, andproduction time of the molding process. Therefore, a need exists for acassette that is not as readily effected by the above listed warpfactors during the plastic injection molding and cooling process.

To permit automated processing of the wafers, the cassette must beindexed with the process tool. An H-bar is formed on the outside of astandardized cassette, which aids in the indexing of the cassette withthe process tool. However, over time the H-bar warps and does notconsistently index a cassette in the same location relative to theprocess tool. Therefore, the H-bar is not a dependable method forattaining high repeatability in indexing the cassette to the processtool.

During automated processing, the indexing of the cassette with theprocess tool must be performed in a clean environment to preventcontamination. In present manufacturing systems, the SMIF pod or boxmust be opened and the cassette removed in such a way that requiresadditional process tools and steps. The increase in required steps andtools adds to the likelihood that the process tool equipment will beunable to interface with the necessary removal tools.

The increase in required steps also requires a greater cycle time toindex a cassette with the process tool. Further, when using a SMIF podor box, the loading height of the process tool equipment must be largeenough to allow for the removal of the standard cassette from the box orpod. Consequently, the SMIF pods or boxes are large and heavy requiringmore storage space and increasing the likelihood of carpal tunnel damageto the handlers of boxes or pods. These problems are all overcome byeliminating the need to use SMIF boxes or pods. Elimination of the boxesor pods also reduces the amount of necessary clean-up, thereby furtherreducing the cost of processing semiconductor wafers.

The present invention overcomes the disadvantages of the currentmanufacturing system by providing a hermetically sealed cassette thatcan be purged with an inert gas. This sealed cassette may be indexeddirectly onto the process tool, eliminating the following steps: openingthe SMIF box or pod, lowering the pod or box door simultaneously withthe cassette, and manipulating the cassette onto the process tool. Thehermetically sealed cassette has one surface of a three-groove kinematiccoupling to positively locate the cassette with the process tool. Thiscoupling provides a dependable method to position the cassette relativeto the process tool with a high rate of repeatability.

Further, the cassette is smaller and about 20% to 50% lighter than theSMIF box or pod. Hence, there is a reduced likelihood that handlers willsuffer from carpal tunnel. The reduced size eliminates the need thatloading height of the process tool equipment be large enough for theremoval of the standard cassette from a SMIF box or pod. Also, therequired storage, transportation, and positioning space are reduced. Atracking system provided on the outer surface of the hermetically sealedcassette allows the cassette to be tracked in an unfriendly externalenvironment during the transportation, storage or processing of thesemiconductor wafers.

The negative effects of warpage, including the increased likelihood ofsemiconductor wafer damage, can be eliminated by providing a uniquecontour to each divider or recess-forming pair. Also, the addition ofexterior ribs integrally corresponding with each contouredrecess-forming pair reduces warpage of the contoured recess-forming pairduring the plastic injection molding and part cooling process. Thecontoured recess-forming pair has a volume dependent on thecorresponding exterior ribs, the cassette sides thickness, the distancebetween each recess-forming pair, and the volume of the perimeter lip.

SUMMARY OF THE INVENTION

The purpose of the present invention is to provide a sealable, purgable,cassette that is compatible with SEMI standards, and having a reducedtendency for the recess-forming pairs (dividers, or slot-forming spacermeans) to warp during the plastic injection molding process. Thecassette has at least one open end, a cover with a sealing means, ameans for supporting the semiconductor wafer's within the cassette, apurging means, a positive indexing means, a tracking means, and ribsextending from the exterior sides of the cassette. The cover whenengaged forms a hermetic seal with the cassette. The cassette isdesigned to securely support semiconductor wafer's within the sealedcassette. The recess-forming pairs which support the wafers arecontoured to help eliminate the negative effects of warpage of therecess-forming pairs during the plastic injection molding process. Thecontoured recess-forming pairs also improve the ability of apre-programmed robotic arm to transfer the wafers in and out of anindexed cassette without damaging or destroying the semiconductorwafers. The hermetically sealed cassette allows direct indexing tosemiconductor wafer processing equipment without the influence ofexternal factors on the contents of the container. The positive indexingmeans provides a dependable method of indexing the cassette to theprocess tool. The indexing means positions the cassette relative to theprocess tool in a precise manner with a high rate of repeatability. Whensealed, the purging means allows a small volume of inert gas to beslowly purged into the cassette, providing a clean environment for thesemiconductor wafers. Tracking means are provided for tracking thecassette without the need for a clean external environment. The exteriorribs are positioned on the outer sides of the cassette and integrallycorrespond to contoured recess-forming pairs. The volumes of thecontoured recess-forming pairs and the ribs are dependent on each othersvolume, the thickness of the cassette sides, the distance between eachrecess-forming pair, the volume of the lip forming the opening to thecassette and the cassette bottom frame. To determine volume dependency,the area of the cassette can be broken into a plurality of smaller equalsegments. Likewise, each element of the cassette has a plurality ofcorresponding segments. Hence, the volumes in the corresponding segmentsof the contoured recess-forming pairs and the ribs are dependent on eachothers segment volume, and all adjacent segment volumes. The volumedependency of each segment shifts the center of mass for each segment ofthe sidewall towards the center of the sidewall. This shift in thecenter of mass for each segment further prevents the contouredrecess-forming pairs from warping during the plastic injection moldingand cooling process.

It is accordingly a principal object of the present invention to providea hermetically sealed cassette which meets SEMI standards.

Another object of the present invention is to provide a simple flexible,low-cost, non-obtrusive cassette for semiconductor wafers, that allowsisolation control.

Another object of the present invention is to provide a purgablecassette.

Another object of the present invention is to provide a cassette thatcan be directly indexed with a process tool in a mini-clean environmentreducing the cycle time to process a semiconductor wafers.

Another object of the present invention is to provide a directlyindexable cassette having a means to positively index the cassetterelative to the process tool with a high rate of repeatability.

Still another object of the present invention is to provide a cassettethat can reduce the number of process tools and steps for semiconductorwafer processing.

A further object of the present invention is to provide a cassette thatenhances reliability during wafer processing by eliminating the need forSMIF boxes or pods, large clean rooms, and the additional processingsteps that the use of boxes or pods requires.

Yet another object of the present invention is to provide a cassettethat reduces the overall weight and size of the package used to storeand transport the semiconductor wafers in a clean environment.

Another object of the present invention is to provide a cassette with atracking system that can be implemented outside of a clean environment.

Still a further object of the present invention is to provide a cassettewhich improves the ability of the robotic arm of automated processingequipment to remove and replace semiconductor wafers from a cassette.

Another object of the present invention is to provide a semiconductorwafer cassette that decreases the likelihood of damage or destruction tothe wafer by the robotic arm during the wafer insertion or extractionprocess.

Another object of the present invention is to provide a cassette havingrecess-forming pairs (slot-forming spacer means or divider) forsupporting and dividing semiconductor wafers that do not warp as readilyduring the plastic injection molding and cooling process.

Another object of the present invention is to shift the center of massof the cassette sidewall towards the center of the sidewallcross-section.

Yet another object of the present invention is to provide a cassettehaving exterior ribs that help prevent the recess-forming pairs fromwarping during the plastic injection molding and cooling process.

These and other objects, as well as the features and advantages of thepresent invention, will be readily apparent to those skilled in the artfrom a review of the following detailed descriptions of the preferredembodiment in conjunction with the accompanying drawings and claims.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of the cassette with the cover and optionalbottom aligned with the cassette.

FIG. 2 is a perspective view of the semi-standard cassette showing anenclosed bottom and the H-bar side.

FIG. 3 is a sectional view of the cassette of the type shown in FIG. 2,showing the inner side of the H-bar, with the alternate preferredindexing spheres shown.

FIG. 3-A is a cross-sectional view through line A--A of FIG. 3.

FIG. 4 is a sectional view of the cassette of the type shown in FIG. 1,with the cover and optional bottom engaged and a wafer positioned withinthe cassette.

FIG. 4-B is a cross-sectional view through line B--B of FIG. 4, with thecover and bottom elevated from the cassette.

FIG. 5 is a sectional end view of the cover of the type shown in FIG. 1,with the seal removed.

FIG. 6 is an end view of the cover of the type shown in FIG. 1, with theseal removed.

FIG. 7 is a perspective view of the cassette, showing the cover andoptional bottom aligned with the cassette, a semi conductor waferpartially extending into the cassette and the optional handle andtracking means, affixed to an end of the cassette.

FIG. 8 is a sectional view of the alternate preferred embodiment of thecassette, showing the inner side of the H-bar, the contouredrecess-forming pairs, and with the alternate preferred indexing spheresshown.

FIG. 9 is a cross-sectional view through line 9--9 of FIG. 8.

FIG. 10 is a cross-sectional view through line 10--10 of FIG. 8.

FIG. 11 is a cross-sectional view through line 11--11 of FIG. 8.

FIG. 12 is a cross-sectional view through line 12--12 of FIG. 8.

FIG. 13 is a greatly enlarged section, showing the recess forming pairs(dividers or slot-forming spacer means) and slots, of the view in FIG.12.

FIG. 14 is a partial side elevational view of a cassette of the typeshown in FIG. 8, showing the exterior ribs extending from the cassetteside, with the cassette being aligned with a three dimensional grid.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring first to FIG. 1, there is indicated generally the sealablesemiconductor wafer cassette 10, a cassette cover 24, a wafer 50, and anoptional cassette bottom door 18.

The semiconductor wafer cassette 10 has a pair of opposing side walls 12and 13, a pair of end walls 14 and 16, an open top 20, and an optionalopen bottom 58. The cassette cover 24 is designed to close and seal theopen top 20. The bottom door 18 is designed to close and seal theoptional open bottom 58.

Referring to FIGS. 2 and 3 the curved semiconductor wafer supportingsidewalls 12 and 13 extend downward from the two opposing perimeter lipedges 28 and 30 to the cassette bottom frame 26. On the inner surface ofthe curved semiconductor wafer supporting side walls 12 and 13 are aplurality of recess-forming pairs (dividers or slot-forming spacermeans) 44a-t and 46a-t shaped like saw teeth with the apex of eachrecess-forming pair aligned on opposing interior surfaces, best shown inFIGS. 1, 3-A, 12 and 13. The apex of each recess-forming pair forms aslot 45, providing a supporting surface for the semiconductor wafers 50in perimeter contact with the lower portion of the curved sides 12 and13. The inside of the optional bottom door 18 may be recessed to form acontinuous support surface to the wafers 50 (see FIGS. 1 and 4). Thecassette may be constructed with a varying number of recess-formingpairs, of course, without deviating from the invention.

Forming the enclosed ends of the semiconductor wafer cassette 10 are theend walls 14 and 16. The end wall 14 forming an enclosed end of thesemiconductor wafer cassette 10 has an optional processing handle 56which extends relatively perpendicular to the end wall 14 and relativelyparallel to the open top 20 (see FIG. 7). The cassette processing handle56 forms a gripping surface. Also, attached to the end wall 14 is theoptional cassette level tracking system 54, allowing a semiconductorwafer cassette 10 to be tracked during processing, transportation, andstorage. Several types of tracking systems may be used including but notlimited to: infrared encoders, radio frequency transmitters, and barcodes that interact with bar code readers.

Opposite the end wall 14 is the H-bar end 16 that forms the otherenclosed end of the semiconductor wafer cassette 10 (See FIG. 2). AnH-bar 41 extends from the surface of the H-bar end that allows thecassette 10 to be indexed with the processing tool. Referring to FIGS. 3and 3-A an optional plurality of partial spheres 42 may extend from theH-bar end 16 allowing precise, consistent and reliable indexing. Theoptional partial spheres 42 form one surface of a three groove kinematiccoupling.

A three groove kinematic coupling consists of two surfaces. Threespheres arranged to form a triangle are attached to one surface andthree grooves, designed to align and engage the spheres, are formedwithin the other surface. The two surfaces are brought into contact. Thespheres engage with the grooves, precisely orienting the two surfacesrelative to the other. This coupling arrangement allows repeatability ofthe precise orientation on the order of the surface finish of thespheres and grooves. The preferred material for the spheres and groovesis a hard ceramic such as tungsten carbide, silicon nitride or zirconia.Of course other materials may also be used without deviating from theinvention. Either the grooves or spheres may extend from the H-bar endwall 16, to form one surface of the kinematic coupling surface 42. Theother surface is formed on the process tool. This arrangement allowsprecise orientation between the cassette and the process tool. In thealternate preferred embodiment, the partial spheres 42 of the kinematiccoupling may replace the H-bar 41.

Purging bores 53 extend through the H-bar end 16 at predeterminedlocations. A purging means consisting of a plurality of self sealingbreather filters may extend from the end wall 16 centered over thepurging bores 53, allowing the sealed semiconductor wafer cassette 10 tobe purged with particle-free air or an inert gas. In the preferredembodiment, a 0.02 micron filter polytetrafluoroethylene (PTFE) membraneis used. These filters may be sealed within a housing that in turnextends through and seals with the purging bore 53. Other suitablemechanisms can, of course, be used without deviating from the invention.The purging bores 53, optional handle 56, purging means, and trackingmeans 54 may alternatively be attached to the cover 24 or bottom 18 ofthe semiconductor wafer cassette without deviating from the invention.

Referring again to FIGS. 1 and 3, the open top 20 is formed by aperimeter lip 60, a perimeter shoulder 32 that extends perpendicularlydownward from an inner edge of lip 60, and a ledge 34 that extendsperpendicularly inward from the perimeter shoulder 32. The perimetershoulder 32 and ledge 34 forms a first sealing means (see FIG. 4). Theperimeter lip 60 provides a support surface for stacking an invertedcassette that is compatible with SEMI standards.

Referring to FIGS. 4, 4-B, 5, and 6 the cassette cover 24 is designed toclose and seal the open top 20. Recessed around the perimeter of thecover 24 is a groove 82 that engages with a seal 23. The seal 23,preferably made of collapsible elastomer, forms a second sealing means.Of course the seal 23 may be made of other acceptable materials such asplastic or rubber without deviating from the invention. When engaged,the cover 24 rests on the cassette perimeter ledge 34. Engaging thecover 24 with the semiconductor wafer cassette open end 20 engages thefirst and second sealing means creating a hermetic seal.

Without limitation, the cover may be constructed with a thickness aboutthe same as the perimeter shoulder 32 so that when the cover 24 engageswith the cassette 10, a relatively flat top surface is formed. The covermay also have a means for supporting the semiconductor wafers. Thismeans for support may be positioned to align with the apex of eachrecess-forming pair when the cover is engaged with the container. Thesesupports would prevent movement of the wafer during transportation. Thecover may be constructed to engage with other surfaces of the cassette,while still forming a hermetic seal without deviating from theinvention.

The present invention may be constructed with an enclosed bottom or withan open sealable bottom 58. In a cassette 10 with an open sealablebottom 58, cassette bottom inner shoulder 66 extends around the insideof the optional cassette bottom frame 26, forming a third sealingsurface. The optional bottom door 18 has a seal 23 extending around theperimeter, forming a fourth sealing surface. When the bottom door 18 isengaged with the semiconductor wafer cassette bottom inner shoulder 66,a hermetic seal is created between the third and fourth sealing means.When engaged, the bottom 18 rests on the cassette bottom perimeter ledge80.

Best shown in FIGS. 1 and 2 are four vertical edges 36 that form thecorners of the semiconductor wafer cassette 10, providing rigidity andsupport to the cassette. The vertical edges 36 extend upward from thelower portion of each end of the curved sides 12 to the perimeter ledge34.

In the alternate preferred embodiment, shown in FIGS. 8-14, exteriorribs 100a-t extend outwardly from the curved semiconductor wafersupporting sidewalls 12 and 13 (see FIG. 14). The exterior ribs 100a-textend downward from the two opposing perimeter lip edges 28 and 30 tothe cassette bottom frame 26. Each exterior rib corresponds to arecess-forming pair (divider or slot forming spacer means) 44a-t and46a-t. The exterior ribs 100a-t are integrally positioned directlyopposite each recess-forming pair on the exterior surface of thesidewalls 12 and 13 (see FIGS. 9-11). The exterior ribs 100a-t areintegrally positioned to help prevent each individual recess-formingpair from warping during the plastic injection molding process. Forexplanatory and comparative purposes, the cassette may be positionedover a three dimensional grid. The area of the cassette can be brokeninto a plurality of smaller equal segments corresponding to the grid.Hence, each element of the cassette has a plurality of correspondingsegments. Further, each corresponding segment has a volume associated toit. The segmented volumes of different elements (the contouredrecess-forming pairs, the perimeter lip, the side walls, the exteriorribs and the cassette bottom frame) of the cassette 10 may be compared(see FIG. 13).

The overall size of each exterior rib 100a-t is dependent on thecombined total volume of: each recess-forming pair 44a-t and 46a-t, thesidewall 12 and 13 between each recess forming pair, the perimeter lip28 and 30, the cassette bottom frame 26 and the particular polymercompound used. As the combined total volume increases, the overall sizeof each exterior rib must also be increased. Each segment of the ribs100a-t positioned on the grid may be proportionately adjusted as afunction of the increase or decrease in the total volume of the adjacentsegments of the sidewall, recess-forming pair, cassette bottom frame,and perimeter lip. By proportionately adjusting the volume of eachsegment of the rib, the center of mass for the combination of the ribsvolume and all the surrounding adjacent segments is shifted towards thewidth center of the sidewall. This shift in center of mass helps preventwarpage.

Therefore, the addition of the exterior ribs 100a-t, together with theproportionate adjustment of volumes, helps prevent the recess-formingpairs 44a-t and 46a-t from warping during the plastic injection moldingand cooling process. A wafer placed in the slot 45 of a warpedrecess-forming pair will have a wafer tilt. This wafer tilt increasesthe likelihood of the robotic arm of the automated processing tool toeither damage or destroy the wafer. Hence, the warped forming pairrenders the cassette useless. As a result of the addition of theexterior ribs 100a-t, wafer tilt is reduced.

To further prevent the recess-forming pairs from warping during theplastic injection molding process the recess-forming pairs 44a-t and46a-t may be contoured without deviating from SEMI standards. Therecess-forming pairs 44a-t and 46a-t are contoured three dimensionallysuch that the volume of a segment of the recess-forming pair is reducedwhen the total combined volume of all adjacent segments increases (seeFIGS. 8 and 13). For example, the volume of each recess-forming pairsegment decreases the closer the recess-forming pair segment is to theperimeter lip 28 (the top portion of the recess-forming pair) or thecassette bottom frame 26 (the bottom portion of the recess-forming pair)because the total combined volumes of all adjacent segments increases(see FIGS. 8-13). This modification to the recess-forming pairs alsoshifts the center of mass of sidewall 12 and 13 and all adjacentsegments towards the center of the sidewalls width.

To conform to standards, the segments of the recess-forming pairs 44a-tand 46a-t, closest to the horizontal axis of the semiconductor wafer(the middle portion of the recess forming pair), must remain constantresulting in a region of segments that form a relatively flat portion104 of the recess-forming pairs (see FIG. 8). In the alternate preferredembodiment the overall contour of each recess-forming pair has thegeneral features of a modified ellipsoid with a flat region through itsmiddle portions (see FIGS. 8 and 13).

The decreased volume in the recess-forming pair segments closest to theperimeter lips 28 and 30, further allows a robotic arm of the processingtool easier access to the semiconductor wafers. The decreased volume ofthe recess-forming pair segments nearer to the cassette bottom frame 26increases the support surface for the wafer. An increased supportsurface towards the bottom of the cassette provides additional relieffor the wafer. The increased support surface also reduces the likelihoodthat a robotic arm inserting a wafer completely into the slot 45 willdamage or destroy the wafer.

Having described the constructional features of the sealable, purgablesemiconductor wafer cassette 10, the mode of use will now be discussed.With a sealed clean cassette containing semiconductor wafers, theoperator of the wafer processing equipment indexes the sealedsemiconductor wafer cassette 10 directly onto the processing tool. Tocontrol the particles on the exterior surface of the integrated cassette10, the processing equipment should provide a mini-environment thatallows a slight overpressure within the mini-environment to prevent theexterior environment from entering into the clean mini-environment. Themini-environment must also provide good air flow to the processing tool.

The cassette 10 may be indexed horizontally or vertically, depending onthe preferred cassette orientation. Then, a processing tool with asuction and vacuum capability aligns and engages the cassette cover 24.The hermetic seal collapses allowing the process tool to remove thecover 24. Next, all the desired processes may be performed on thesemiconductor wafers within the cassette. After all the desiredprocesses have been performed, the cover 24 is then resealed by engagingthe cover 24 with the cassette's perimeter ledge 34, discontinuing thevacuum and disengaging the suction. A hermetic seal is formed betweenthe perimeter shoulder 32, the ledge 34 and the cover's seal 23.

The cassette may then be transported to other process stations, storedor otherwise transported. During storage, the cassette may be purgedwith an inert gas utilizing two 0.02 micron filter PTFE membranesproviding a clean environment for semiconductor wafers 50. When theactive purge is removed, the sealed cassette 10 will maintain the inertgas purge for several hours. This method allows the semiconductor wafersto be safely stored and transported in an unfriendly externalenvironment. Also, during processing, transportation or storage, theoptional tracking system 54 may be used to track the cassette in anexternal environment. The optional handle 56 on the end wall 14 of thecassette 10 facilitates loading and unloading a cassette into processequipment and storage areas.

This invention has been described herein in considerable detail in orderto comply with the Patent Statutes and to provide those skilled in theart with the information needed to apply the novel principles and toconstruct and use such specialized components as are required. However,it is to be understood that the invention can be carried out byspecifically different equipment and devices, and that variousmodifications, both as to the equipment details and operatingprocedures, can be accomplished without departing from the scope of theinvention itself.

What is claimed:
 1. A cassette for securely containing semiconductorwafers comprising:a container having spaced parallel solid sides havingopposed interior surfaces, an open top end, and including a lipextending peripherally around said open top end, a plurality ofslot-forming spacer means aligned on said opposing interior surfacesforming a plurality of aligned parallel slots to support semiconductorwafers, said slot-forming spacer means are configured to facilitateautomated insertion and removal of wafers supported in said slotswherein a top portion of each slot-forming spacer means near the opentop end is narrower in width than, and extends from one of said interiorsurfaces less than, a relatively lower, middle portion of theslot-forming spacer means.
 2. The cassette as recited in claim 1 furthercomprising exterior ribs extending downwardly from said lip andprotruding outwardly from opposing exterior surfaces of said solid sidesof said container to a cassette bottom frame, whereby each said exteriorrib is opposite and aligned with a said slot-forming spacer means toinhibit warpage of each of said slot-forming spacer means.
 3. A cassettefor securely containing semiconductor wafers comprising:a containerhaving spaced parallel solid sides having opposed interior surfaces, anopen top end, and including a lip extending peripherally around saidopen top end, a plurality of dividers, each protruding inwardly to anapex, on said opposing interior surfaces and forming a plurality ofparallel slots to support semiconductor wafers, and exterior ribsextending downwardly from said lip to a cassette bottom, each ribprotruding outwardly to an apex from an exterior surface of said solidsides of said container, whereby the apex of each of said exterior ribsis substantially aligned with the apex of one of said dividers in adirection perpendicular to the surfaces to inhibit warpage of each ofsaid dividers.
 4. The cassette as recited in claim 3 in which saiddividers are contoured to allow a robotic arm of an automated processingequipment easier access to semiconductor wafers supported by saiddividers wherein a top portion of each divider near the open top end isnarrower in width than, and extends from a said interior surface lessthan a relatively lower, middle portion of the divider.
 5. The cassetteas recited in claim 4 wherein a bottom portion of each divider near thecassette bottom frame is narrower in width than, and extends from saidinterior surface less than said middle portion of the divider.
 6. Thecassette as recited in claim 3 in which said dividers are contoured suchthat said slots do not effect semiconductor wafer transfer when warpageof said dividers occurs wherein a top portion of each divider near theopen top end is narrower in width than, and extends from a said interiorsurface less than, a relatively lower, middle portion of the divider,and a bottom portion of the divider near the cassette bottom frame isnarrower in width than, and extends from said interior surface less thansaid middle portion of the divider.
 7. The cassette as recited in claim3 in which each exterior rib is further comprised such that a segment ofsaid exterior rib is proportionate to a combined volume of surroundingadjacent segments of said solid sides, said dividers, said lip and saidcassette bottom frame in order to shift a center of mass of said ribsegment and all of said surrounding adjacent segments towards a centerof a corresponding segment of said solid sides.
 8. A cassette forsecurely containing semiconductor wafers comprising:a container havingspaced parallel sides having opposing interior surfaces, an open topend, and including a lip extending peripherally around said open topend, a plurality of recess-forming pairs of dividers aligned on saidopposing interior surfaces forming a plurality of parallel slots tosupport semiconductor wafers, and exterior ribs extending downwardlyfrom said lip and protruding outwardly from opposing exterior surfacesof said sides of said container to a cassette bottom frame, whereby eachexterior rib is further comprised such that a segment of said exteriorrib is proportionate to a combined volume of surrounding adjacentsegments of said sides, said dividers, said lip and said cassette bottomframe in order to shift a center of mass of said rib segment and all ofsaid surrounding adjacent segments towards a center of a correspondingsegment of said sides.
 9. The cassette as recited in claim 8 in whichsaid recess-forming pairs are contoured to allow a robotic arm of anautomated processing equipment easier access to semiconductor waferssupported by said recess-forming pairs wherein a top portion of eachdivider near the open top end is narrower in width than, and extendsfrom a said interior surface less than a relatively lower, middleportion of said divider.
 10. The cassette as recited in claim 9, whereina bottom portion of said divider near the cassette bottom frame isnarrower in width than, and extends from said interior surface less thansaid middle portion of said divider.